1. Field of Invention
This discovery concerns the improvement of the reliability and damage tolerance of brittle materials through the use of novel crack arresting architectures composed of compressive layers specifically placed throughout the body that prevent failure from occurring until a specific, predetermined threshold strength is reached.
2. Description of Related Art
The strength of most common brittle materials is not deterministic, i.e. single-valued, due to the presence of an unknown distribution of strength-limiting flaws inadvertently introduced during processing and surface machining [1,2]. As a result, the strength of brittle materials must generally be described by a statistical distribution of strengths with associated probabilities of failure at each of those strengths. Failure from these types of flaws is generally not an issue in ductile materials because they exhibit plastic deformation that desensitizes the relation between small flaws and strength. Plastic deformation also absorbs work from the loading system to increase the material's resistance to the extension of large cracks. However, the lack of plastic deformation in brittle materials causes their strength to be inversely dependent on the size of very small cracks, which generally cannot be detected except by failure itself.
Consequently, design with brittle materials generally becomes a practice of defining acceptable levels of reliability. Designers must not only make accommodations for probabilistic definitions of the strength and the finite probability of failure at any applied stress, but they must also be further concerned with the fact that, once in service, seemingly insignificant and sometimes undetectable damage could be incurred that would drastically reduce the load carrying ability of the material. This lack of reliability is one of the major reasons why brittle materials have not been more widely used, despite the potential they offer for substantial performance enhancements in a wide variety of applications.
One method for improving the reliability of components made from brittle materials has been through the use of proof testing. The proof test is designed to emulate the thermomechanical stresses experienced by the component in severe service and defines a threshold stress below which components are eliminated by failure prior to service. However, given its destructive nature, proof testing is generally only used when performance needs outweigh consumer price sensitivity. In ceramics, another approach to ensuring reliability is by eliminating heterogeneities that give rise to flaws, such as inclusions and agglomerates, from the ceramic powder. One method to remove heterogeneities greater than a given size is to disperse the powder in a liquid and pass the slurry through a filter [1]. If heterogeneities are not reintroduced in subsequent processing steps, and surface cracks introduced during machining are not a critical issue, filtration determines a threshold strength by defining the largest flaw that can be present in the powder and thus, within the finished ceramic component [3]. However, neither of these techniques mitigates the detrimental effect of service-related damage.
Recently, another method for improving reliability through the use of residual, compressive stresses that have their maxima located some specific distance beneath the surface of the material was proposed by Green et al [4]. The authors suggested that the unique compressive stress profiles they developed would arrest surface cracks and lead to higher failure stresses and improved reliability through reduced strength variability. However, compressive stresses, either at or just beneath the surface, will not effectively hinder internal cracks and flaws, nor can they produce a threshold strength; thus high reliability is still not ensured. As shown below, a threshold strength can only arise when compressive layers are placed on the surface and throughout the body to interact with both surface cracks and internal cracks and flaws.